In optical fiber networks, wavelength division multiplexing (WDM) uses a device known as a wavelength division multiplexer to multiplex individual optical signals into a single multiplexed signal that a single optical fiber can carry. WDM is generally used when the number of fibers in an existing transmission link is inadequate or designing a system with a sufficient number of fibers becomes cost prohibitive.
WDM systems often operate at frequencies other than the frequency for which an existing transmission link may best operate. For example, some WDM systems may operate at optical frequencies of between 1540 and 1550 nm, while the existing transmission link may be designed to array a signal having a 1310 nm wavelength. For these instances, undesirable optical chromatic dispersion may occur in the optical path. The optical fiber path for a WDM system, therefore, may require chromatic dispersion compensation to achieve desired performance characteristics.
Optical fiber compensation is usually selected to reduce the chromatic dispersion to zero at a wavelength near the planned wavelength of operation. This wavelength is called the dispersion-zero or zero-chromatic-dispersion wavelength. In some types of dispersion compensation, both positive and negative residual chromatic dispersion, may persist at wavelengths other than the dispersion-zero wavelength. This remaining chromatic dispersion may limit the usefulness of the optical fiber transmission path, because the non-zero dispersion may compromise or preclude transmission of optical signals having frequencies other than the dispersion-zero frequency.
C. Lin, H. Kogelnik, and L. G. Cullen, "Optical-Pulse Equalization of Low-Dispersion Transmission in Single-Mode Fibers in the 1.3-1.7 .mu.m Spectra Region," Optics Letters, v.5, No. 11 (Nov. 1980), describes an optical-pulse-equalization technique for minimizing pulse dispersion in a single-mode fiber transmission system. The technique uses the positive and the negative dispersion characteristics of single-mode fibers on both sides of a dispersion-zero wavelength. While the technique is successful in controlling chromatic dispersion for a single wavelength, it does not address the problem of providng compensation over a band of wavelengths.
It is therefore an object of the present invention to provide a method and system to simultaneously and independently control chromatic dispersion of independent optical signals that are to be wavelength division multiplexed for transmission.
It is an object of the present invention to provide a system that permits simultaneous and independent control of optical dispersion at any desired wavelength to increase the usable wavelength range of optical signals for wavelength division multiplexing. To achieve these results, the present invention individually compensates optical dispersion of all signals associated with a wavelength division multiplexer. This is in addition to the dispersion compensation that the multiplexed signal itself experiences. The method and system undercompensates the multiplexed optical signal by a predetermined amount so that each of the optical signals has an associated residual optical dispersion. To eliminate the residual optical dispersion, each of the optical signals is compensated individually to produce a zero optical dispersion along each individual optical path that will receive the demultiplexed optical signals. Consequently, each of the optical signals that make up the multiplexed optical signal has minimal chromatic dispersion upon receipt.
It is also an object of the present invention to permit modular interchanging or replacement of different optical signal transmitters while maintaining the same performance along the wavelength division multiplexed signal path. By selecting individual compensation to match the precise wavelength of the individual transmitter and, then, including the individual residual dispersion compensation as a modular part of the optical signal transmitter itself, the present invention permits non-disruptive interchanging or replacement of dispersion compensated transmitters.